1. Field of the Invention
The present invention relates to a feedback control circuit, and more particularly to a feedback control circuit using an integrating circuit or a high frequency eliminating circuit.
2. Description of Related Art
Generally, the control that compares an output with a reference value and minimizes the difference between the two is know as PID (proportional-plus-integral-plus-derivative) control in the automatic control (for example, see, Shigeru Kurosu, “Introduction to Control Engineering” pp. 77–86, 2nd edition, POWERSHA Inc. May 30, 1994). In the field of electric circuits, the PID control is applied, for example, to AGC (Automatic Gain Control) (see, Green, D. “Global stability analysis of automatic gain control circuits” Circuits and Systems, IEEE Transactions on, Volume 30, Issue 2, February, 1983, pp. 78–83, for example), or to a PLL (Phase-Locked Loop) (see, Larsson, P. “Measurements and analysis of PLL jitter caused by digital switching noise” Solid-State Circuits, IEEE Journal of, Volume 36, Issue 7, July, 2001, pp. 1113–1119, for example).
Although the AGC or PLL uses an integrator or low-pass filter in the feedback control circuit, the output of the integrator or low-pass filter is limited within the power supply voltage of the circuit. Recently, in particular, the power supply voltage has been reduced with the scale down of circuit elements from the viewpoint of the withstanding voltage, and hence the ranges of the output voltages of circuits have also been reduced.
The control circuit controls the system in response to the output of the integrating circuit or low-pass filter. Accordingly, when the output range of the integrating circuit or low-pass filter is narrow, the control circuit must increase the gain to cover the required control range. Increasing the gain, however, presents a problem of greatly changing the state of the system because of noise added to the output of the integrating circuit or low-pass filter.
In addition, the integrator or low-pass filter includes a high impedance node, which is sensitive to noise. In addition, an external component such as a capacitor attached to the integrator or low-pass filter is subjected to circulate noise on a board. Even if all the components are integrated into an IC, the circulate noise from a substrate cannot be eliminated completely, and hence the integrator or low-pass filter is subjected to influences of the noise.
The present invention is implemented to solve the foregoing problems. It is therefore an object of the present invention to provide a noise-resistant feedback control circuit capable of suppressing the influence of noise even when the output of the integrating circuit or low-pass filter undergoes the noise by configuring the control system without increasing the gain after the integrating circuit or low-pass filter under the limited power supply voltage range as the output range the integrating circuit or low-pass filter.